This invention relates to a sprayed plug for use in a piercing and elongating mill, more particularly a plug having an excellent durability and utilized in piercing mills.
A plug is used for a piercing and elongating mill adapted to manufacture seamless steel pipes. Heretofore, such plug has been prepared by casting an alloy steel containing 0.3% by weight of carbon, 3% by weight of chromium and 1% by weight of nickel, heating the steel alloy to a temperature of 900.degree..about.950.degree. C. and then cooling. In a Mannesmann piercing mill, a heated steel piece is rolled between opposed rolls which are inclined with respect to the axis of the plug at the same time the plug is pushed into the central portion of the steel piece to enlarge the central opening, thus obtaining a pipe having desired inner diameter. Since the plug is brought into slide contact with the steel piece heated at a temperature of about 1200.degree. C., it suffers extensive damage such as wear, abrasion and deformation so that its durability or number of uses is low. Damaged plug forms scratches on the inner surface of the pipe so that it is necessary to exchange the plug before it becomes badly damaged. Accordingly, it is necessary to carefully and frequently inspect the plug which requires much time and labour. Where the plug is fixed to a mandrel rod, time and labour are required to exchange the damaged plug thus decreasing productivity. As an example of an improved plug having increased durability, an alloy steel containing 0.2% by weight of carbon, 1.6% by weight of chromium, 0.5% by weight of nickel, 1.25% by weight of cobalt and 1% by weight of copper has been proposed. However, this alloy is not economical because it contains copper and cobalt. Especially, cobalt is not stably available because of its poor resources. Moreover, all prior art plugs have been heat treated to form an oxide scale thereon. While the oxide scale provides heat insulation and a lubricating function between the heated steel piece and the body or core of the plug, as has been clearly pointed out in U.S. Pat. No. 3,962,897 the oxide scale can not exhibit sufficiently large heat insulation and lubrication functions where the steel piece has a tendency of entrapping the slag. To obviate this problem, there has been proposed a plug made of a cobalt base heat resisting alloy not formed with the oxide scale. The plug made of such a cobalt base steel alloy is not only expensive but also the experiment made by the inventors showed that it does not always have high durability. Although this type of plug is not formed with an oxide scale, as it is subjected to a solid solution heat treatment and an aging heat treatment its manufacturing cost is high.
FIG. 1 of the accompanying drawing shows one example of the damage of a prior art plug which is used for a Mannesmann piercing mill. Thus, wear 11 and pealing-off 12 are formed at the fore end, while wrinkles 13 or cracks 14 are formed on the body portion. The wrinkles 13 are formed due to the insufficient high temperature strength, while the cracks 14 are formed due to the thermal stress and the insufficient toughness. The wear 11 and peeling-off 12 are caused by wearing away of the surface scale thereby causing seizure. For this reason, it has been practically difficult to obtain a plug having improved durability and free from such damages caused by different causes. Consequently, a low alloy steel containing 0.3% by weight of carbon, 3% by weight of chromium and 1% by weight of nickel, for example, has been preferred. The wrinkles 13 or cracks 14 shown in FIG. 1 are caused by a rise in the surface temperature. For this reason, these defects can be eliminated if an oxide scale having a sufficiently large heat insulating property could be formed. And example of such improvement is disclosed in Japanese laid open patent application No. 17363/1979. According to the method disclosed therein the heating atmosphere utilized to form the oxide scale is controlled by admixing water therewith so as to form a stable oxide scale. With this method, however, the plug is not improved to maintain adequate balance among the shape, heat insulating property and lubricating property of the oxide scale, and the mechanical characteristics of the base metal alloy can not withstand piercing conditions which are becoming severer with year.